A pellicle is a piece of transparent material mounted at a distance from the absorber surface on a reticle in the direction of the wafer. The distance is chosen to be out of focus from the wafer, so that any foreign particles fallen on the surface of the pellicle is out of focus on the wafer, thus not contributing to printing defects.
The thickness of a pellicle is preferred to be small so that its presence in the optical path does not affect the performance of the imaging lens. It is usually in the order of a few micrometers. Therefore, only polymer films can be found to have satisfactory optical and mechanical properties for pellicle materials. Polymer materials have been available as the imaging wavelength was reduced from 436 to 365, 248 and 193 nm.
When the imaging wavelength is further reduced to 157 nm, there is difficulty in developing a thin and durable soft material that has sufficient light transmission. One has to use pellicles which is much thicker and rigid. With a thickness sufficient to satisfy the mechanical requirement, it is already affecting the imaging property of the lens. Therefore, the flatness, smoothness and parallelism of a thick pellicle have to be of high optical quality, making it very expensive.
Another drawback in using either soft or hard pellicles in 157 nm lithography is the requirement of an oxygen-free transmission space. The entire optical path in a 157 nm imaging tool is flushed with high-purity nitrogen to prevent oxygen from getting into it. The pellicle either has to permanently seal nitrogen in its enclosed volume with the mask or has to allow for quick replacement of air with nitrogen when it is loaded in the imaging tool. Either case is not easy to implement.
It is therefore an object of the present invention to provide a mask that is protected by a pellicle which does not have the drawbacks or shortcomings of the conventional mask/pellicle assembly.
It is another object of the present invention to provide an apparatus for mounting a hard pellicle to a mask wherein the hard pellicle has optical transmission properties at least those of quartz.
It is a further object of the present invention to provide an apparatus for mounting a hard pellicle to a mask in an environment filled with a light-transmitting gas so that the interior cavity between the mask and the hard pellicle is light-transmitting.
It is still another object of the present invention to provide an apparatus for mounting two hard pellicles to a mask with one on each side of the mask.
It is still another object of the present invention to provide an apparatus for mounting at least one hard pellicle to a mask which includes delivery means for delivering the at least one hard pellicle covered mask to a scanner.
It is yet another object of the present invention to provide an improved projection mask aligner in which a mask having a protective hard pellicle mounted thereon is positioned in-between a light source and an imaging lens.